An optical network interface unit (NIU) comprises an enclosure that houses equipment for converting optical signals on an incoming fiber optic cable into electrical signals that can be used by a local network. In a fiber-to-the-home (FTTH) environment, the NIU marks the division between the outside fiber plant controlled by a service provider and the customer-owned network wiring. Typical NIU's thus include a first input for receiving a fiber optic cable and a second input for receiving an electrical conductor, often a coaxial cable. Also included in the NIU is an optical-to-electrical converter for converting optical signals received on the fiber to electrical signals that can be transmitted over a coaxial cable. This may include, for example, a photodiode that outputs electrical signals in response to received optical signals. The NIU may also include a laser for converting electrical signals received from the home network into optical signals and transmitting those optical signals upstream over the fiber optic cable.
It is known to use a single fiber optic cable for transmitting optical signals from a head end to a home and for transmitting optical signals from the home back to the head end. The direction from the head end to the home may be referred to herein as the “downstream” direction, and the direction from the home to the head end may be referred to as the “upstream” direction. In order to do this, the downstream transmissions typically take place using a first wavelength or band of wavelengths and upstream transmissions take place using a second wavelength or band of wavelength spaced from the first band of wavelengths. The downstream transmissions may be produced by one of a small number of relatively expensive lasers that are maintained in a temperature-controlled environment in the head end, which lasers can transmit in a relatively narrow band of wavelengths without significant drift. Such transmissions may take place for example, at a particular wavelength in a band of wavelengths from about 1550 to 1560 nanometers. Upstream transmissions are produced by less expensive lasers which may experience significant drift in the band of wavelengths in which they transmit. This drift may be increased if the upstream transmitting laser is mounted in an outdoor NIU and subject to temperature fluctuations, because the temperature at which some lasers operate affects their transmission wavelength. These transmissions nominally take place in a band of frequencies near 1310 nanometers, but which may be as broad as 1260 to 1360 nanometers to accommodate variations in the wavelengths of the transmitting lasers. It is also known, alternately, to send upstream transmissions in the 1565 to 1625 nanometer band of wavelengths, above the band of wavelengths often used for downstream transmissions.
The downstream optical signals received at the NIU may include wavelengths outside the band of wavelengths intended for the optical-electrical converter. For example, erbium doped fiber amplifiers (EDFA's) in a transmission system sometimes produce wideband noise that may reach the NIU. Fiber backscatter may also occur in a typical downstream transmission. Reflection points in a network, such as connectors in the system, may reflect the backscattered light in a forward direction toward the NIU. Some of this light may be directed toward the receiver in the NIU if the reflected signal falls within the range of wavelengths removed by the filter. Other portions of this reflected signal can reach the laser in the NIU, and this unwanted signal impinging on the laser may degrade the performance of upstream laser transmissions. It would therefore be desirable to reduce the effects of unwanted downstream optical signals on the NIU and in particular, on the laser in the NIU, without interfering with downstream transmissions intended for the receiver in the NIU.